RI 


8749 



Bureau of Mines Report of Investigations/1983 




Selective Nickel Electrowinning 
From Dilute Electrolytes 



By G. R. Smith, W. R. Thompson, 
and P. E. Richardson 




UNITED STATES DEPARTMENT OF THE INTERIOR 



Report of Investigations 8749 



Selective Nickel Electrowinning 
From Dilute Electrolytes 



By G. R. Smith, W. R. Thompson, 
and P. E. Richardson 




UNITED STATES DEPARTMENT OF THE INTERIOR 
James G. Watt, Secretary 

BUREAU OF MINES 
Robert C. Horton, Director 



,l('|2 



*^* 



This publication has been cataloged as follows: 



Smith, Gerald R., 1940- 

Selective nickel electrowinning from dilute electrolytes. 

(Report of investigations ; 8749) 

Bibliography: p. 20. 

Supt. of Docs, no.: I 28.23:8749. 

1. Nickel— Electrometallurgy. 2. Electrolytes. I. Thompson, W. R. 
(William Richard), 1957- . II. Richardson, Paul E. III. Title. 

IV. Series: Report of investigations (United States. Bureau of 
Mines) ; 8749. 

-W2-M^43~ [TN799.N6] 622s [622'. 348] 82-600223 









CONTENTS 

Page 



Abstract 1 

Int r oduc t ion 2 

Experimental 3 

*" Channel cell 3 

Equipment 4 

Vj Preparat ion of solut ions 4 

Synthetic electrolyte 4 

Leach electrolyte 5 

Deposit evaluation 5 

Results 6 

Synthetic electrolyte 6 

Electrochemical studies 6 

Deposit evaluation 10 

5 g/1 Ni 2+ electrolyte 10 

1 g/1 Ni 2+ electrolyte 13 

Leach electrolyte 13 

Leaching 13 

Purification 15 

Nickel electrowinning 16 

Conclusions 18 

References 20 

ILLUSTRATIONS 

1 . Schematic of channel cell and flow system 3 

2. Velocity profiles for channel cell 4 

3. Top view of channel cell with I-V measuring system 5 

4. I-V curves "at several velocities — 5 g/1 Ni 2+ , 50° C 7 

5. Tafel slopes— 5 g/1 Ni 2+ , 50° C 8 

6. Velocity versus limiting current density — 5 g/1 Ni 2+ , 50° C 9 

7 . Arrhenius plot — 5 g/1 Ni 2+ 9 

8. I-V curves at several velocities — 1 g/1 Ni 2+ , 50° C 10 

9. Cross section photomicrographs of nickel deposits — 5 g/1 Ni 2+ , 

50° C 11 

10. SEM photographs of nickel deposits — 5 g/1 Ni 2+ , 50° C 12 

11. Photomicrographs and SEM photographs — 1 g/1 Ni 2+ , 50° C 14 

12. Pressure leaching curves for gabbro ore flotation concentrate .... 15 

13. Photomicrograph of electrowon copper 15 

14. Fluidized bed apparatus for copper removal 16 

15. Copper removal rate through fluidized bed of nickel powder 16 






*9* 



TABLES 



Page 



1. Composition of gabbro ore flotation concentrate 6 

2. Nickel electrowinning — dilute leach electrolyte 17 

3. Nickel electrowinning — dilute leach electrolyte, deposit composi- 
tion versus electrolyte concentration 18 

4. Nickel electrowinning — dilute leach electrolyte, velocity effect 

on deposit composition 18 





UNIT OF MEASURE ABBREVIATIONS USED 


IN THIS REPORT 


A/cm 2 


ampere per square 
centimeter 


lim 


micrometer 






mm 


millimeter 


A/m 2 


ampere per square 








meter 


Mohm-cm 


megohm-centimeter 


° C 


degree Celsius 


m/s 


meter per second 


cm 2 /s 


square centimeter 
per second 


mv/s 


millivolt per second 






ohm-cm 


ohm-c e nt ime t e r 


g/1 


gram per liter 










pet 


percent 


g/ton 


gram per ton 










ppm 


part per million 


hr 


hour 










psi 


pound per square inch 


K 


kelvin 










rpm 


revolution per minute 


kcal/mo 


le kilocalorie per mole 










V 


volt 


kw 


kilowatt 


1 








v (SHE) 


volt, standard hydrogen 


1 


liter 




electrode 


1/min 


liter per minute 


wt-pct 


weight -per cent 


min 


minute 







SELECTIVE NICKEL ELECTROWINNING FROM DILUTE ELECTROLYTES 

By G. R. Smith, ' W. R. Thompson, 2 and P. E. Richardson 3 



ABSTRACT 



Critical and strategic metals are often present in 
small quantities in low-grade domestic ores. When these 
ores are leached, the resulting solution usually contains 
the metals in very dilute quantities. Selective 
e lee t r owinning from dilute electrolytes was investigated by 
the Bureau of Mines. A metal deposit containing 84 pet 
nickel was electrowon from the leach solution of a complex 
domestic ore bulk flotation concentrate originally contain- 
ing approximately 2 wt-pct nickel. Key to achieving accele- 
rated deposition rates, in the case of dilute solutions, is 
the rapid movement of the electrolyte through the electro- 
winning cell. Pure, synthetic nickel electrolytes were used 
to establish deposition parameters, and to optimize electro- 
lyte velocity or mass transfer rates. Hydrodynamic and 
e lee t r odepos i t ion parameters for both the synthetic and 
leach electrolytes are presented and the experimental re- 
search described. Selective e lee t r owinning appears to offer 
a viable alternative to physical separation methods to 
obtain separate metal "concentrates" from low-grade ores. 

^Supervisory research chemist. 

•'Chemist . 

-'Supervisory research physicist. 

All authors are with the Avondale Research Center, Bureau of Mines, 

Avondale, Md. 



INTRODUCTION 



Increasing the relative motion of 
the electrolyte with respect to the 
electrode surface has long been recog- 
nized as a means by which the mass 
transport of ions to the electrode can 
be increased to allow operation at 
higher current densities. Basic 
theories and early experiments were 
discussed by Nernst ( 12) ,^ Brunner ( 4_) , 
Lin, Denton, Gaskill, and Putnam (11), 
Eisenberg, Tobias, and Wilke (6), and 
Levich ( 10 ) . 

High mass transport cells also per- 
mit electrowinning from dilute solu- 
tions. The ability to electrowin metals 
from dilute solutions acquires particu- 
lar significance as a result of recent 
trends to use hydrometallurgical proc- 
esses as a means to avoid pollution 
problems associated with smelting and to 
recover metals from low grade ores by in 
situ leaching. Increasing interest has 
also been shown by the electroplating 
industry in conserving metal values 
ordinarily lost through "drag out" of 
solution on the plated parts. Barker 
and Plunkett (2) studied the electro- 
lytic recovery of nickel at concentra- 
tions of 0.001 to 0.02 g/1 Ni 2+ in both 
planar electrode and fluidized bed elec- 
trode systems. Bettley, Tyson, 
Cotgreave, and Hampson (3) evaluated a 
combination forced flow, fluidized bed 
electrowinning cell as a method for re- 
covering nickel from dilute electroplat- 
ing effluents having concentrations of 
0.3 to 1.25 g/1 Ni 2+ . Landau (9_) inves- 
tigated the distribution of copper ion 

^Underlined numbers in parentheses re- 
fer to items in the list of references 
at the end of this report. 



transport rates along planar electrodes 
at concentrations of 0.32 to 6.3 g/1 
Cu 2+ and electrolyte flow rates up to 
4.6 m/s (Reynolds number (Re) = 
100,000). Kovacs (8) studied the mass 
transfer and hydrodynamic phenomena on 
planar electrodes during the electro- 
winning of copper from 3 g/1 Cu 2 + 
solution circulated at 9.2 m/s. Skarbo 
and Harvey (14) and Harvey, Miguel, Lar- 
son, and Servi (!) applied air agitation 
to increase mass transport in copper 
electrowinning from dilute solutions and 
showed that a current density of 32 A/m 2 
was possible for each gram per liter of 
copper in solution. 

The principal objectives of the re- 
search described in the present report 
were (1) to establish the hydrodynamic 
and electrodeposit ion parameters for 
electrowinning nickel in a channel cell 
utilizing synthetic, dilute (1 to 5 g/1 
Ni 2+ ) electrolytes, and (2) to evaluate 
electrolyte purification procedures re- 
quired to electrowin nickel in a channel 
cell from an actual dilute leach elec- 
trolyte. The latter solution was pre- 
pared from a bulk sulfide flotation con- 
centrate of Duluth gabbro ore which con- 
tained significant concentrations of Cu, 
Ni , and Fe ; and lesser quantities of Co, 
Pb , Zn, and precious metals. Studies 
with this concentrate and related leach 
solutions represented an introduction of 
the metal recovery process at an early 
stage in the mineral processing proce- 
dure. Successful implementation of a 
process to recover metals from dilute 
solutions by electrowinning is an alter- 
native to pyrometallurgical processing 
and hydrometallurgical solvent extrac- 
tion stages. 



EXPERIMENTAL 



CHANNEL CELL 3 

The channel cell used in this study 
is shown schematically in figure 1. 
Electrolyte was circulated through the 
cell with a 95 1/min positive dis- 
placement pump from either a 30- or a 
225-1 iter-capacity reservoir corre- 
sponding to the use of either leach or 
synthetic, dilute solution. A 400- mm- 
long, 200-mm-diam cylindrical chamber 
preceded the electrolysis channel and 
served to minimize turbulence eddies 
associated with the transition from the 
32-mm-diam connecting pipe and the 
1 , 200-mm-long, 13-mm-wide, 50-mm-high 
channel electrolysis section. The 
cylindrical chamber contained three 
successively finer screens (8, 14, and 
20 mesh) positioned perpendicular to the 
flow to act as manifolds for distribut- 
ing the flow evenly across the entrance 
to the channel. A shorter chamber (200 
mm) containing no screens was positioned 
after the exit from the channel . Each 

^obert Associates, Inc., Bowie, Md . , 
assisted in the design and hydrodynamic 
characterization of the channel cell. 



end of the channel was tapered and 
projected about 75 mm into the adjacent 
cylinder to further aid in minimizing 
eddy effects. Electrolyte velocity 
through the test cell was computed from 
the volumetric flow that was monitored 
with a meter in the return section of 
the circuit. The rate of flow was 
adjusted with in-line valves as well as 
with a bypass circuit that also con- 
tained a gas venting chamber. Velocity 
profiles in the channel were measured 
with a laser-Doppler anemometer (19). 
Figure 2 shows two velocity profiles 
measured at a cross section 20 hydraulic 
diameters from the entrance and at mid- 
height. They represent linear flow 
rates of 0.10 and 1.4 m/s corresponding 
to Reynolds numbers of 3,000 and 43,000, 
respectively. The corresponding shape 
of each curve is similar to that which 
is typically obtained under fully 
developed laminar and turbulent flow 
conditions. Velocity distribution 
across the channel becomes more uniform 
with increasing turbulence because of 
decreasing viscous effects and increas- 
ing inertia forces in the bulk solution. 



Immersion heater 



/Bypass 



! i i ! I i J 



#1 



Pump 



/ 



IS 



Gas vent 
chamber 



- Flowmeter 



Screens 

EL 



Reservoir 



ir/ 



Electrodes 



"Z 



5^ 



_& 



Entrance^ 
cylinder 



Channel cell 



> 






FIGURE 1. - Schematic of channel cell and flow systerru 




Horizontal center of channel - 



DISTANCE FROM CATHODE, mm 

FIGURE 2. - Velocity profiles for channel cell. 
Velocity measured at cross section 20 hydraulic 
diameters from entrance and at midheight. 

The electrolytic cell was con- 
structed of 25-mm-thick polyacrylic 
plastic. The lid was connected to the 
cell body with steel reinforced poly- 
acrylic clamps, and sealed with a 
neoprene 0-ring gasket. Materials of 
construction for the cylinders and the 
bypass chamber were 6-mm-thick 
polyacrylic and PVC, respectively. 

Quartz immersion heaters (up to 3.75 
kw) were mounted within the reservoir to 
thermostatically control the electrolyte 
at desired temperatures up to 50° C. 



opposite that of the cathode. The 
Luggin capillary passed through this 
wall and a small hole in the Pb-6Sb 
anode. Three individual nickel 
cathodes, 20 mm high and 45 mm long, 
were mounted in the sidewall 20, 30, and 
40 hydraulic diameters from the channel 
entrance. This permitted three 
independent potent iodynamic sweep 
measurements or electrolytic tests to be 
made simultaneously at a chosen velo- 
city. Fully developed flow is generally 
accepted to exist between 25 and 75 
hydraulic diameters. Three separate 
anodes were mounted on the opposite wall 
and each had a surface area 50 pet 
larger than the cathodes. The edges of 
the removable cathodes were sealed with 
a rubber based filler. Electrical 
connection to the cathodes and anodes 
was made through 3.2-mm-diam brass rods 
threaded into the back of the elec- 
trode . 

All potent iodynamic sweep measure- 
ments were made at a position near the 
center of the cathode (22 mm downstream 
from the leading edge) and represented 
only an average polarization in view of 
the fact that the convective diffusion 
profile changes along the length of an 
electrode in a flowing electrolyte. 

Electrolysis circuits for the three 
electrode pairs consisted of a constant 
current power supply, a shunt for cur- 
rent measurements, and an ampere-hour 
meter. Anode-cathode and cathode- 
calomel potentials were measured with 
high input impedance voltmeters. 

PREPARATION OF SOLUTIONS 



EQUIPMENT 



Synthetic Electrolyte 



Figure 3 shows a detailed schematic 
of the electrolytic cell configuration 
and associated electrochemical measuring 
system. The reference electrode was 
connected via a salt bridge to a 0.5-mm- 
diam Luggin capillary probe positioned 
1.3 mm from the nickel 200 alloy cathode 
surface. Accurate positioning of the 
probe was accomplished by means of a 
micrometer mounted on the outside wall 



Synthetic dilute nickel electrolytes 
were prepared using reagent grade salts 
and deionized water ( p =10 Mohm-cm) . 
Nickel sulfate hexahydrate was used to 
supply the nickel ions in concentrations 
ranging from 1 to 5 g/1 Ni^ + . The sup- 
porting electrolyte consisted of 35 g/1 
sodium sulfate (Na2S04) and 20 g/1 boric 
acid (H3BO3). 






Luggin capillary 




J 



L^WV-^r Cathode 



:M 



K\\\\^ .^ X\\\\<i^_ Anod ; 




Channel — ' Sidewall - 



Capillary 
positioning device - 




"Brass connection 



u 



Salt bridge- 



Saturated 
/ calomel 



'.'KJ 



-• * 



Potentiostat 



Voltage programer 



FIGURE 3. - Top view of channel cell with l-V measuring systertit 



Leach Electrolyte 

To prepare a representative leach 
solution, a bulk flotation concentrate 
of the Duluth (Minn.) gabbro ore obtain- 
ed from the Twin Cities (Minn.) Research 
Center was leached following closely the 
procedure of Vezina (16). The concen- 
trate was composed primarily of 
pyrrhotite (FeS) and chalcopyrite 
(CuFeS2), with lesser amounts of 
pentlandite (FeNi)qSg. Its elemental 
composition is listed in table 1. The 
concentrate was ground to 100 pet minus 
20 mesh, mixed with a 20 g/1 H 2 S0 4 , 20 
g/1 H3BO3, 35 g/1 Na 2 S04 solution to a 
pulp density of 17 pet solids, and then 
leached in an autoclave, under 100 psi 
oxygen pressure at a temperature of 110° 
C, for 5 to 8 hr. H3BO3 and Na 2 S04 were 
utilized in the leaching procedure to 
provide supporting electrolyte in subse- 
quent electrowinning experiments. The 
concentrate and solution were continous- 
ly mixed during leaching by means of 75- 



mm-diam stainless steel paddles rotated 
at 575 rpm. Decomposition reactions 
that occur during leaching are expressed 
by equations 1 through 3 (16) . 

The postleaching procedure included 
shutting off the external oxygen pres- 
sure, slowly releasing the internal 
autoclave pressure to the vapor pressure 
of the solution, cooling to -60° C, and 
finally disassembling and filtering the 
resulting slurry through acid resistant 
paper in a Buchner type funnel. 

DEPOSIT EVALUATION 

Cathodes were prepared for deposi- 
tion by polishing with 240-grit alumina 
cloth, rinsing with water, wiping with 
ethyl alcohol, and drying in air. At 
the completion of an electrolysis 
period, nickel deposits were removed 
from the cell, washed in water, air 
dried, and weighed to obtain current 
efficiency values. For physical 



(FeNi) 9 S 8 + 4.50 2 + 9H 2 S0 4 + 4.5NiS0 4 + 4.5FeS0 4 + 8S° + 9H 2 (1) 

FeS + 0.50 2 + H 2 S0 4 ->• FeS0 4 + S° + H 2 (2) 

CuFeS 2 + 2 + 2H 2 S0 4 + CuS0 4 + FeS0 4 + 2S° + 2H 2 (3) 



evaluation, the deposits were stripped 
from the cathode and examined using 
photomicrography and scanning electron 
microscopy (SEM). For determining grain 
structure and the sizes and distribu- 
tions of voids, cross sections were cut 
from the geometrical center of the de- 
posits, mounted in clear plastic, sanded 
to a 600-grit surface, polished with 20- 
then 5-)jm alumina powder, and finally 
etched at 50° C in a sulfuric acid- 
hydrogen peroxide solution for 5 to 10 
min. Photomicrographs were obtained at 
magnifications up to 600. SEM 
examinations were used to evaluate the 
uniformity of deposit growth, 
crystalline size, and evidence of the 
initiation of nodular formations. 



TABLE 1. - Composition of gabbro ore 
flotation concentrate 



Component, wt-pct : 



Concentration 



Iron 37.9 

Sulfur 26.6 

Copper 12.1 

SiO 10.0 

A1 2 3 3.7 

Nickel 2.1 

CaO 1.7 

MgO 1.7 

TiO 15 

Cobalt 11 

Lead 10 

Zinc 06 

Manganese .04 

Cadmium .02 

Component, g/ton: 

Silver 30.51 

Palladium 2.40 

Gold 68 

Platinum 31 



RESULTS 



SYNTHETIC ELECTROLYTE 

Electrochemical Studies 

In order to determine the effect of 
electrolyte flow rate on the electro- 
chemical parameters for nickel deposi- 
tion from dilute solutions, slow (5 
mv/s) potentiodynamic sweeps from the 
open-circuit potential to approximately 
the hydrogen evolution potential were 
carried out at a number of flow velo- 
cities between and 2.1 m/s. The elec- 
trolytes contained either 5 or 1 g/1 
Ni^ + , and the temperature was 50° C. 

Immediately prior to the potentio- 
dynamic sweeps, a thin coating of nick- 
el was deposited on the blank nickel 200 
alloy cathode to ensure that the result- 
ing polarization curves would be repre- 
sentative of Ni-Ni^" 1 " deposition reac- 

"Experimental data were obtained by 
Carl Goldsmith, engineering technician, 
and William Kolodrubetz, physical sci- 
ence aid, Avondale Research Center, 
Avondale, Md. 



tion. Following this procedure, the 
open-circuit potential still remained 
somewhat unstable, varying between 
and 0.1 v (SHE). This instability can 
probably be attributed to the lack of a 
truly reversible equilibrium for the Ni- 
Ni2+ couple and to the existence of 
other electrochemical reactions such as 



Ni0 2 + 4H + + 2e~ = Ni 2+ + 2H 2 (4) 

and 2 + 4H + + 4e _ =£ 2H 2 0, (5) 

so that the open-circuit assumed a mixed 
potential value. 

Potentiodynamic current-voltage ( I- 
V) curves for a nickel electrolyte con- 
taining 5 g/1 Ni 2+ are shown in figure 
4. Similar results were obtained at all 
three electrode locations along the 
length of the cell. Each curve has been 
corrected for the resistive voltage ( IR) 
component of the electrolyte using a 
measured resistivity of 19.53 ohm-cm and 
a Luggin probe distance of 1.3 mm. 



1,500 



1,200 



900 



600 



300 




-0.4 -0.6 -0.8 

CATHODE POTENTIAL, v (SHE) 

FIGURE 4, - l-V curves at several velocities (meters 
per second)-5g/l Ni 2 +, 50 u Ct Letter designations in- 
dicate hydrodynamic and electrodeposition parameters 
for nickel deposits shown in figures 9 and 10. 



The onset of nickel deposition occurs at 
0.65 to 0.75 v. The thermodynamic 
potential for nickel deposition at this 
concentration is =-0.31 v (5_) , and the 
relatively large overvoltage (0.34 to 
0.44 v) can probably be attributed to 
a large activation overvoltage (2). 

The I-V curve for zero flow exhi- 
bits a well-defined limiting current 
density (ij) of approximately 100 A/m^ 
extending from --0.65 to -0.9 v as 
expected for deposition under diffusion 
limited conditions. At potentials more 
negative than -0.9 v, the rise in 
current density corresponds to hydrogen 
evolution, as evidenced by visible 



gaseous evolution at the electrode sur- 
face . 

With flow rates in the laminar range 
(0.05 and 0.10 m/s), somewhat less 
defined plateaus are observed. Above 
flow rates of 0.10 m/s, the current 
density over the diffusion limited 
region develops a finite slope that 
increases with increasing velocity. 
Finite slopes over this region also 
have been observed for copper deposition 
in channel type cells (9, 13). This 
characteristic of the diffusion limited 
region has been attributed to a larger 
current density (thinner diffusion 
layer) towards the leading edge of the 
electrode than towards the trailing 
edge. Under controlled potential con- 
ditions, this current density gradient 
along the electrode obscures the plateau 
normally associated with a diffusion 
limited current. 

Analysis of the I-V curves using the 
relationship n = a + b log i (where ri is 
overvoltage, a and b are Tafel con- 
stants, and i is current density) for 
the 5 g/1 Ni2 + electrolyte at velocities 
of 0.85, 1.4, and 2.1 m/s (fig. 5) 
yields a Tafel slope (b) of - 0.13, an 
exchange current density (ig) of -5 X 
10~-> A/cm , and a transfer coefficient 
(a) of =0.24. These values are 
reasonably close to values reported in 
the literature (18). 

At the onset of the diffusion limit- 
ed region (fig. 4), the deposition rate 
is controlled by the flux of nickel ions 
to the electrode surface. This flux in- 
creases with increasing velocity owing 
to a thinning of the diffusion layer 
and, thus, increases the range of cur- 
rent densities where nickel can be depo- 
sited. At low velocities, in the range 
of laminar flow, the limiting current 
density (ij) an< i the diffusion layer 
thickness (6) for deposition onto a 
plate electrode at a position x from the 
leading edge of flow are given (1) by 

i L (x) = (l/3)ZFU 1/2 v- 1/6 D 2/3 Cx- 1/2 (6) 

and 6(x) = 3x 1/2 tf 1/2 v 1/6 D 1/3 , (7) 



UJ 

O 

< 
l- 

_i 

o 

> 

UJ 

> 
O 



0.6 
.5 
.4 
.3 
.2 

.1 



I I 


i i 


KEY 

— • 0.85m/s 
■ 1.40 m/s 
▲ 2.10m/s 








• 


/ ■ 

A 


y 

I 





-5 -4 -3 -2 -1 

LOG i, A/cm 2 

FIGURE 5. - Tafel slopes-5 g/l Nj2+, 50° C. 

where U is the velocity of the electro- mental values of ii( x ) were found to 
lyte parallel to the electrode plane at depend on U*'^ and to extend signi- 



an infinitely large distance from it, v 
is the kinematic viscosity of the elec- 
trolyte, D is the diffusivity, C is the 
concentration of nickel ions in the 
bulk solution, Z is the number of elec- 



ficantly into the turbulent region. 

Using equation 6 and the slope 
obtained from figure 6, the diffusivity 
value for the nickel ion was calculated 



trons transferred, and F is the Fara- to be 3.7 X 10"^ cm^/s. A diffusivity 

value of 3.9 X 10"^ cm^/s was obtained 
using a rotating disk electrode, further 
substantiating the reliability of the 
experimental i T (x) values even though 
clearly defined limiting current 
plateaus were not observed with increas- 
ing velocity. 



day constant. 

Figure 6 shows a plot of the 
experimental ij( x ) values versus U^'^ 
where the ij(x)'s represent those ob- 
tained at the onset of the diffusion 
limited region (fig. 4). The laminar 
and turbulent flow regions are defined 
in terms of Reynolds number (Re) which 



In another series of potent io- 



is related to factors such as cell dynamic sweeps with the 5 g/l Ni^ + 

electrolyte, the activation energy (E^) 
for the deposition of nickel was deter- 
mined from the I-V curves at 30°, 35°, 
and 40° C using a velocity of 0.85 m/s. 
The effect of temperature on the rate 
(i) of an electrochemical reaction is 
expressed by the Arrhenius equation 



geometry, flow rate, and kinematic 
viscosity of the solution. Re values 
under 3,000 are generally considered to 
be indicative of laminar flow, whereas 
Re values above 11,000 are considered 
to be characteristic of fully turbulent 
flow. A mixed transition region exists 
between these two values. The experi- 



ln i = (E A -ZaHF)/RT, 



(8) 



REYNOLDS NUMBER 
11,000 



43,000 




FIGURE 6. - Velocity versus limiting current den- 
sity-5 g/l Ni 2+, 50° C 



and E» must be determined at constant 
overvoltage (n). An Arrhenius plot (In 
i verses 1/T) at a constant overvoltage 
of 0.41 v, assuming a shift of -0.01 v 
in reversible electrode potential from 
30° to 40° C (5), shows the expected 
linear relationship (fig. 7) and yields 
an activation energy of =19.8 kcal/mole. 
A value of 21 kcal/mole has been re- 
ported for a 0.5M NiS04 electrolyte uti- 
lizing reversible potential data ob- 
tained at 25° and 45° C U8) . 

Potent iodynamic I-V curves for a 
nickel electrolyte containing 1 g/l 
Ni 2+ with a thermodynamically reversible 
potential of =-0.33 v (_5) are shown in 
figure 8. Similar results were again 
obtained at all three electrode 
locations. Each curve was also correct- 
ed for the IR component of the electro- 
lyte (resistivity = 18.80 ohm-cm). A 
well-defined limiting current density 
plateau was observed only at zero flow 
and the onset of nickel deposition was 




3.14 



3.34 



3.24 
1/Tx10 3 , K 

FIGURE 7. - Arrhenius plot-5 g/l N i 2^ 50° C. 



essentially in the same voltage range as 
for the 5 g/l Ni 2 + electrolyte. 
Analysis of the I-V curves for the 1 g/l 
Ni 2+ electrolyte at several velocities 
yielded a Tafel slope of 0.13, an 
exchange current density of - 3 X 10 - ^ 
A/ cm 2 , and a transfer coefficient of 
"--0.24. 

The Tafel region extended to current 
densities of 1,400 and 200 A/m 2 for 5 
and 1 g/l Ni 2+ electrolytes, respec- 
tively, using the maximum flow rate of 
2.1 m/s. Accuracy of the exchange 
current density value determined for 
these electrolytes is limited by the 
accuracy of the Tafel slope and the 
reversible potentials at each Ni 2+ con- 
centration. There is some uncertainty 
in the Tafel slope because of the 
existence of some diffusion control and 
codeposition of hydrogen in the Tafel 
region for nickel deposition. 



10 




CATHODE POTENTIAL, v (SHE) 

FIGURE 8, - l-V curves at several velocities (meters 
per second)— 1 g/l Ni 2 +, 50°C. Letter designations in- 
dicate hydrodynamic and electrodeposition parameters 
for nickel deposits shown in figure 11. 

Deposit Evaluation 

5 g/l Ni 2+ Electrolyte 

Nickel electrodeposits were prepared 
for evaluation at flow rates and current 
densities corresponding to the letters A 
through F on the I-V curves (fig. 4). 
Cross section photomicrographs of the 
respective deposits are shown in figure 
9. Excellent deposits exhibiting a 
coarse-grained columnar structure 
typical of sulfate electrolytes were 
obtained using deposition conditions 
where essentially no mass transport 
limitation of the nickel ions existed, 
that is, the deposition region 
characterized by the sharp increase in 
current density near -0.65 v. 

Deposits B, D, and F (240 A/m 2 , 0.15 
m/s), (530 A/m 2 , 0.60 m/s) , and (800 
A/m 2 , 1.40 m/s), respectively, 
correspond to deposits prepared in this 



region. Deposit A produced at 320 A/m 2 
and 0.15 m/s was nonconsolidated and 
exhibited large voids and nodular 
formations, reflecting growth in the 
limiting current region. Deposit C 
produced at 750 A/m 2 , 0.60 m/s was a 
more compact deposit than A containing 
small voids. It was also produced in 
the mass transport limiting region but 
under less stringent polarization con- 
ditions. Current efficiencies for 
deposits prepared in the mass transport 
limiting and the activation controlled 
deposition regions ranged from 80 to 95 
pet . 

The nonconsolidated character of 
deposit E, (1,400 A/m 2 , 1.40 m/s) indi- 
cated that it was prepared at condi- 
tions outside of the activation con- 
trolled region, although the transition 
from activation control to mass trans- 
port limiting conditions was difficult 
to detect from the I-V curves. Appar- 
ently, at this high flow rate the 
gradient in current density along the 
length of the electrode almost totally 
obscures the diffusion limited current. 
SEM photographs of the surface of 
deposits A, C, E, and F are shown in 
figure 10. Deposit F shows the expected 
smooth, consolidated surface whereas 
deposits A, C, and E show varying 
degrees of nodular formation and crystal 
growth. 

In addition to the poor physical 
quality of deposit E, the current effi- 
ciency decreased to 65 pet, indicating 
that the overvoltage for hydrogen 
evolution decreases with increasing flow 
rate. Decreases in current efficiency 
were also observed for deposits prepared 
at 450 and 600 A/m 2 (not designated on 
figure 4) when the flow rate was 
increased from 0.85 to 1.40 m/s, 
although for these deposits the micro- 
structure remained consolidated and 
columnar . 

Yeager (17) also reported a decrease 
of 0.35 v in the overvoltage for 
hydrogen evolution on a nickel substrate 
when ultrasonic waves were used to 
agitate solution in the region near the 



11 








f w*-*m^mmi^^mmmm 










FIGURE 9. - Cross section photomicrographs of nickel deposits— 5 g/l Ni 2 +, 50 C\ 



12 







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w 








f 

■ ■ 




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i» 


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FIGURE 10. - SEM photographs of nickel deposits-5 g/l Ni 2+, 50° C. 



13 



electrode surface. In comparison, there 
appeared to be a decrease of =0.075 v in 
the overvoltage for hydrogen evolution 
when the velocity was increased from 
to 2.10 m/s (fig. 4) in the high mass 
transport channel cell. 

Numerous theories have been advanc- 
ed regarding the rate-determining mech- 
anism of hydrogen gas discharge at a 
solid electrode surface. These include 
the combination of atomic hydrogen into 
H2 molecules, the formation of hydrogen 
bubbles, the diffusion of H + to the 
electrode, as well as adsorption- 
desorption phenomena associated with 
hydrogen atoms, molecules, and ions. 
The kinetics of one or all of these 
mechanisms could be affected in the 
transition from free convection to 
laminar to turbulent flow in the elec- 
trowinning cell, but it is beyond the 
scope of this report to discuss these 
kinetic factors. However, the practical 
significance of a decrease in hydrogen 
overvoltage is to limit the range of 
acceptable deposition currents. 

1 g/1 Ni 2+ Electrolyte 

Electrodeposits were also produced 
for physical evaluation from an elec- 
trolyte containing only 1 g/1 Ni 2+ . 
Photomicrographs and SEM photographs 
for two electrodeposits produced at G 
and H (fig. 8) are shown in figure 11. 
These deposits were prepared using a 
velocity of 1.40 m/s. Deposit G, pro- 
duced in the activation controlled de- 
position region at 210 A/m 2 was well 
consolidated while deposit H, produced 
at conditions where significant mass 
transport limitation existed (450 
A/m 2 ), was poorly consolidated showing 
large voids and dendritic growth. A 
decrease in current efficiency from 76 
to 68 pet was observed as the current 
density was increased from 210 to 450 
A/m 2 . 

The rates of deposition of satis- 
factory nickel deposits from a 1 and 5 
g/1 Ni 2+ electrolyte flowing at 1.40 
and 0.15 m/s, respectively, are similar 
to those obtained for industrial nickel 



electrowinning from a 60 g/1 Ni 2+ elec- 
trolyte operated under essentially sta- 
tic conditions. 

LEACH ELECTROLYTE 

After establishing the hydrodynamic 
and electrodeposition parameters for 
producing satisfactory nickel deposits 
from synthetic electrolytes, the results 
were used to test the feasibility of 
utilizing the high mass transport cell 
to electrowin nickel from a dilute leach 
solution. These experiments involved 
(1) preparing a bulk quantity of leach 
electrolyte by pressure leaching, (2) 
removing some of the metal ion 
impurities from the electrolyte using 
appropriate purification procedures, and 
(3) conducting preliminary nickel 
electrowinning experiments with the 
resultant leach liquor. 

Leaching 

Typical leaching results for Ni, Cu, 
and Fe are shown in figure 12. In a 7- 
hr leaching test, 80 pet of the nickel 
and approximately 20 pet of the copper 
were leached. As acid was consumed 
during leaching the pH increased from 1 
to 1.7. Correspondingly the dissolved 
Fe 2+ was oxidized, hydrolyzed, and 
precipitated as ferric oxide 
(Fe203 'x^O) , which decreased the 
iron content of the solution to 0.3 
g/1. Using a solution- to -ore -concen- 
trate weight ratio of 5, the nickel and 
copper leached from the ore corresponded 
to concentrations of 3.4 and 4.8 g/1, 
respectively. Between 30 and 40 percent 
of the Co, Zn, and Mn was also dissolved 
during this leaching period, as well as 
2 to 5 pet of the Ag, Pb , and Cd . 

After combination of several leach 
liquors and dilution with Na2S04~ H3BO3 
solution to obtain the desired volume of 
45 1, the final concentration of elec- 
trolyte was, in g/1: 1.0 Ni, 1.5 Cu, 
0.084 Fe, 0.05 Co, 0.06 Zn, 0.01 Mn, 
0.004 Pb, 0.003 Cd, and 0.0001 Ag. 



14 







FIGURE 11. - Photomicrographs and SEM photographs-1 g/l Ni2+, 50° C. 






15 




2 4 6 

RETENTION TIME, hr 

FIGURE 12. - Pressure leaching curves for gabbro 
flotation concentrate. 

Purification 

Although the leach solution obtained 
from the complex bulk flotation concen- 
trate contained several metallic ion 
impurities, studies to remove iron and 
copper prior to nickel electrowinning 
experiments were given particular atten- 
t ion . 

Controlled potential electrowinning 
was studied as a method for removal of 
copper from the leach solution. The 
channel cell system was utilized with 
the only modification being an increase 
in electrode size for increasing the 
total current through the cell. One 
large titanium cathode and an equally 
large Pb-6Sb anode spanning the height 
and length of the electrolysis section 
replaced the three independent smaller 
electrode pairs used for studies with 
synthetic electrolyte. An electrolyte 
flow rate of 1.40 m/s was maintained. 

Sixty-five percent of the initial 
1.5 g/1 Cu in the leach solution was 
removed by pot ent iostat ical ly elec- 
trowinning (-0.28 v (SHE)) at an average 
current density of 120 A/m^ and a cath- 



ode current efficiency of 80 pet. The 
physical quality of the electrowon cop- 
per was excellent (fig. 13), and the 
purity of the copper was about 99.9 pet. 
The deposit contained 25 ppm (0.0025 
pet) Ag. Copper remaining in solution 
was reduced to 0.003 g/1 in an addition- 
al electrolysis at an average current 
density of 48 A/m^, but the quality of 
this material was dark and powdery and 
it was deposited at much lower current 
efficiencies. 




FIGURE 13.- Photomicrograph of electro- 
won copper. 

The iron impurity remaining in solu- 
tion after leaching was removed by a 
subsequent procedure that consisted of 
oxidation of the iron with 02> air, or 
another suitable oxidizing agent such as 
H2O2 , adjustment of the pH to 3.5 with 
calcium carbonate or caustic, and 
finally heating the solution to 85° C 
to effect a more complete hydrolysis and 
precipitation. The leach electrolyte 
initially containing 0.084 g/1 Fe was 
decreased to 0.020 g/1 Fe using this 
procedure. Some hydrolysis and 
coprecipitat ion of nickel also occurred, 
resulting in a 7-pct decrease of nickel 
in solution. 

In addition to the purification 
steps conducted on the leach solution, 
associated tests were conducted with a 
synthetic solution to establish a rapid, 
efficient method for removing low con- 
centrations of copper. A solution con- 
taining 5.0 g/1 Ni and 1.0 g/1 Cu was 
decreased to 0.0002 g/1 Cu after 30 
cycles of the solution through a fluidi- 
zed bed (fig. 14) of active nickel pow- 
der ( ^100 pm) , using a flow rate of 
0.003 m/s. A weight ratio of 10:1 
nickel powder to total copper was 



16 



employed. Most of the copper was re- 
moved after 17 cycles as illustrated in 
figure 15. 

The removal of small quantities of 
lead was accomplished using controlled 
potential electrolysis in the channel 
cell. Electrodeposits containing 70 pet 
Pb were obtained at a potential of -0.6 
v from a synthetic solution containing 3 
g/1 Ni and 0.005 g/1 Pb flowing at a 
velocity of 1.40 m/s. The potential for 
lead deposition was about 0.05 v less 
electronegative than for deposition of 
nickel. The flow of electrolyte in- 
creased the limiting current density for 
lead deposition so that a practical 
current density of 20 A/rn^ could be 
obtained . 



; j s 



- Filter cloth 
-Fluidized bed 



Flowmeter - 




Nickel Electrowinning 

Electrowinning experiments were 
conducted to recover nickel from the 
dilute leach electrolyte that had been 
purified of copper and iron to the 
levels of 0.003 and 0.02 g/1, respec- 
tively. 

After a few preliminary electrolyses 
an electrodeposit containing 84 pet Ni 
was attained from a solution containing 
0.75 g/1 nickel ion (table 2). Major 
impurities were Zn, Co, Cu, and Fe. The 
cathode current efficiency for Ni reduc- 
tion was -65 pet. This deposit was pro- 
duced at 200 A/m2 using an electrolyte 
flow rate of 1.40 m/s. These conditions 
were similar to those established for 
satisfactory deposition from a synthetic 
electrolyte containing 1.0 g/1 Ni. 




10 20 

CYCLES THROUGH FLUIDIZED BED 



FIGURE 14. - Fluidized bed apparatus for copper 
removal. 



FIGURE 15. - Copper removal rate through flu- 
idized bed of nickel powder. 



17 



TABLE 2. - Nickel electrowinning — 
dilute leach electrolyte 





Electrolyte 


Electrodeposit 


Metal 


composition, 


composition, 




g/1 


pet 


Nickel .... 


0.75 


84.3 


Zinc 


.03 


4.2 


Cobalt 


.023 


2.9 




.020 


2.7 




.007 


.5 


Manganese . 


.005 


.15 


Copper 


.003 


2.7 


Cadmium. . . 


.0002 


.02 



Deposits produced at a lower current 
density (100 A/m 2 ) and a higher velocity 
(2.10 m/s) contained 30 pet Ni , 15 pet 
each of Pb and Zn, and less than 1 pet 
of each of Cd, Co, Fe and Mn. 

The total metal content in each of 
these deposits was less than 100 pet. 
The balance was apparently related to 
the coprecipitation of a yellow solid 
during electrowinning. Recovery of a 
sample of this material and subsequent 
analysis by proton induced X-ray emis- 
sion showed it to contain about 40 pet 
Ni , 15 pet Fe , and a substantial quan- 
tity of O2 (likely a mixed oxide). 

In a continuing series of electro- 
winning experiments with the leach solu- 
tion, further studies were conducted to 
determine the deposition rate of the 
remaining impurities. To conduct these 
experiments the concentration of the 
nickel ions was increased from 0.75 to 
2.80 g/1 by addition of nickel sulfate 
hexahydrate . 

Results of nickel electrowinning 
experiments at 300 A/m 2 with this leach 
solution predictably showed, under 
constant hydrodynamic conditions (0.85 
m/s), that the rate of codeposition of 
impurities decreased nearly propor- 
tionally with the concentration of 
impurities in solution (table 3). Pro- 
gressive purification of the solution 
and gradual deposition of higher purity 
nickel would likely occur in subsequent 
electrolysis stages; however, an 
optimum nickel purity would not be 



attainable by this method until the 
nickel ion concentration in solution 
also had decreased significantly. 

Results in table 3 also show that 
coprecipitation of oxides again occurred 
during these electrolyses as evidenced 
by the metal content of less than 100 
pet. Apparently the amount of nickel as 
oxide was greater in the first deposit 
but the reason for this difference was 
not established. It is assumed that ad- 
ditional measures must be taken to in- 
sure adequate buffering near the elec- 
trode surface. This would prevent 
locally high pH regions where the nickel 
ion might tend to hydrolyze and precipi- 
tate. 

Since mass transport of impurities 
as well as nickel is affected by the 
hydrodynamics of the solution, several 
experiments were conducted at 200 A/m 2 
to determine the effect of velocity on 
the impurity deposition when electro- 
lyzing at conditions established to be 
in the range of satisfactory nickel 
deposition (table 4). When the velocity 
was increased from 0.85 to 1.40 m/s, the 
percentage of the more electropositive 
impurities, Pb, Cu, and Cd increased in 
the deposit by 6, 5.5, and 4 times, 
respectively, while that of iron 
remained essentially unchanged. The 
percentage of zinc, a more electro- 
negative metal not normally expected to 
codeposit, actually increased by a fac- 
tor of 2.5 as the velocity was 
increased. The presence of zinc ions 
increases the overvoltage for nickel 
deposition by as much as 0.3 v, leading 
to coreduction of zinc and nickel ions 
(15). Apparently this increase in over- 
voltage is due to the adsorption of a 
zinc hydroxide layer on the cathode. 
Zinc hydroxide may actually shift the 
deposition potential for nickel slightly 
more electronegative than that for zinc, 
since the zinc deposition rate increased 
with velocity in a manner similar to the 
more electropositive impurities. These 
results suggest that with careful 
control of the cathodic potential and 
the electrolyte velocity, a more 
selective removal of zinc could be 
achieved . 



18 



TABLE 3. - Nickel electrowinning — dilute leach electrolyte, deposit 
composition versus electrolyte concentration 





1st deposit 


2d deposit 


Metal 


Electrolyte 

composition, 

g/1 


Electrodeposit 

composition, 

pet 


Electrolyte 

composition, 

g/1 


Electrodeposit 

composition, 

pet 


Nickel .... 
Cobalt 

Manganese . 

Copper .... 
Cadmium. . . 


2.80 
.025 
.020 
.006 
.005 
.004 
.003 
.0002 


76.0 

1.91 

2.79 

.80 

.32 

1.41 

.17 

.005 


2.35 
.019 
.015 
.003 
.004 
.003 
.0006 
.0001 


91.0 
1.62 
1.39 
.38 
.27 
.57 
.07 
.002 



TABLE 4. - Nickel electrowinning — dilute leach electrolyte, 
velocity effect on deposit composition 





Electrolyte 

composition, 

g/1 


Electrodeposit composition, pet, at — 


Metal 


0.85 m/s 


1.40 m/s 


Manganese . . 
Cadmium. . . . 


2.79 
.028 
.025 
.007 
.004 
.003 
.001 
.001 


81.1 
1.72 
2.31 
.33 
.59 
.80 
.005 
.16 


53.4 

1.17 

5.60 

.16 

.54 

4.81 

.02 

.90 



Some codeposition of manganese was 
observed even though the thermodynamic 
potential for Mn^ + reduction is an 
additional 0.12 v more electronegative 
than for the coreduction of zinc and 
nickel. This may result from potential 
gradients on the electrode surface of 
sufficient magnitude to deposit small 
quantities of manganese. Deposition of 
manganese decreased by 52 pet as the 
velocity was increased from 0.85 to 
1.40 m/s. 

The overvoltage for cobalt deposi- 
tion is probably increased by the pres- 



ence of zinc ions in a manner similar to 
nickel. According to the results in 
table 4, the percentage of cobalt in the 
electrodeposit was decreased by 32 pet 
as the velocity was increased, indicat- 
ing its retention in solution at proper- 
ly controlled potentials and velocities; 
thus allowing for its subsequent re- 
covery. Cobalt could, of course, be re- 
moved by precipitation as cobaltic hy- 
droxide after oxidation with chlorine 
or nickelic hydroxide, a common practice 
in purifying nickel electroref ining 
solutions . 



CONCLUSIONS 



Electrochemical data for nickel 
deposition from dilute solutions, in- 
cluding Tafel constant, exchange current 



density, transfer coefficient, and acti- 
vation energy values, compared closely 
with values reported in the literature. 



19 



Limiting current density plateaus are 
not clearly defined on the I-V curves at 
increasing velocities due to a current 
density gradient along the length of the 
electrode which obscures the plateau. 
However, using the value obtained at the 
onset of the diffusion limited region as 
the limiting current density value, it 
was observed to vary as the square root 
of the velocity significantly into the 
turbulent region. 

Hydrodynaraic and electrodeposition 
parameters were established for produc- 
ing suitable nickel electrodeposits from 
pure dilute (1 to 5 g/1 Ni) electrowinn- 
ing type solutions, utilizing a high 
mass transport channel cell. Cathode 
current efficiencies for favorable de- 
posits ranged from 75 to 95 pet. Excel- 
lent nickel electrodeposits, free of 
voids and exhibiting a columnar crystal- 
line structure, were obtained at =j 210 and 
800 A/m 2 for 1 and 5 g/1 Ni electrolytes, 
respectively, using an electrolyte flow 
of 1.40 m/s. Corresponding current ef- 
ficiencies were 76 and 83 pet. 

Electrowinning studies conducted 
with an impure dilute leach solution 
prepared from the Duluth gabbro ore 
flotation concentrate yielded an elec- 
trodeposit containing 84 pet Ni . In re- 
lated electrolyte purification studies, 
copper and iron were removed effectively 
from the leach solution to levels as low 
as 0.003 and 0.020 g/1, respectively, 
using electrowinning and precipitation 
techniques . 



Selective electrolytic removal of 
lead ions was also demonstrated to be 
possible using a closely controlled 
cathodic potential and a rapid flow of 
electrolyte to increase mass transfer of 
lead ions. At an electrode potential of 
-0.6 v, deposits containing 70 pet Pb 
were obtained using a synthetic electro- 
lyte. 

The presence of zinc ions in solu- 
tion reportedly increases the overpoten- 
tial for nickel electrowinning by ==0.3 v 
causing the codeposition of significant 
quantities of zinc. Electrolytic remov- 
al of zinc from the leach solution was 
increased by a factor of 2.5 as the 
electrolyte flow rate was increased from 
0.85 to 1.40 m/s, suggesting that, with 
careful control of the cathodic poten- 
tial and the electrolyte velocity, a 
more selective removal of zinc impurity 
would be achieved. The deposition rate 
for removing the more electropositive 
impurities, Pb, Cu, and Cd , increased by 
factors of 4 to 6 when the velocity was 
increased from 0.85 to 1.40 m/s. 

Electrolyte purification and nickel 
electrowinning studies demonstrated the 
feasibility of applying dilute solution 
electrowinning technology to the hydro- 
metallurgical processing of complex low- 
grade ore concentrates. Advances in the 
technology for electrowinning from di- 
lute solutions would provide a method 
whereby direct recovery of metals could 
be accomplished at a very early stage in 
the mineral processing procedure. 



20 



REFERENCES 



1. Antropov, L. I. (Theoretical 
Electrochemistry). Mir Publishers, 
Moscow, 1st ed . , 1972, translated by A. 
Beknazarov, 568 pp. 

2. Barker, B. D., and B. A. Plunkett. 
The Electrolytic Recovery of Nickel From 
Dilute Solutions. Trans. Inst. Metal 
Finish., v. 54, pt . 2, 1976, pp. 104- 
110. 

3. Bettley, A., A. Tyson, S. A. 
Cotgreave, and N. A. Hampson. The 
Electrochemistry of Nickel in the 
Chemelec Cell. Surface Technol . , v. 12, 
1981, pp. 15-24. 

4. Brunner, E. Reaction Velocity in 
Heterogeneous Systems. Z. Physik. 
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5. Carr, D. S. , and C. F. Bonillo. 
II. Nickel in Neutral Sulfate Solution. 
J. Electrochem. Soc . , v. 99, No. 12, 
1952, pp. 475-481. 

6. Eisenberg, M. , C. W. Tobias, and 
C. R. Wilke . Ionic Mass Transfer and 
Concentration Polarization at Rotating 
Electrodes. J. Electrochem. Soc., v. 
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7. Harvey, W. W. , A. H. Miguel, P. 
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Air Agitation in Electrolytic Decopperi- 
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v. 84, Sect. C, 1975, pp. 11-17. 

8. Kovacs, L. Diffusional Mass 
Transfer on High Specific Surface 
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Veszprem, Hungary, 1977, pp. 31-37. 

9. Landau, U. Distribution of Mass 
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10. Levich, 
Hydrodynamics 



V. G. Physicochemical 
Prentice-Hall, Englewood 



Cliffs, N.J. , 1962, 700 pp. 

11. Lin, C. S., E. B. Denton, H. S. 
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2136-2143. 

12. Nernst , W. Theory of Reaction 
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13. Selman, J. R. Measurement and 
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Ph.D. Thesis, Univ. Calif., Berkeley, 
Calif., 1971, 304 pp. 

14. Skarbo, R. R. , and W. W. Harvey. 
Conditions for the Winning of Copper in 
the Form of Coherent High-Purity Elec- 
trodeposits. Trans. Inst. Min and Met., 
v. 83, Sect. C, 1974, pp. 213-222. 

15. Vaaler, L. E. Electrolytic Puri- 
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J. Electrochem. Soc. Electrochem. Sci. 
and Technol., February 1978, pp. 204- 
207. 

16. Vezina, J. A. Acid Pressure 
Leaching a Pentlandite-Chalcopyrite- 
Pyrrhotite Concentrate. Dept . of 
Energy, Mines and Resources, Ottawa, 
Canada, Mines Branch Tech. Bull. 129, 
1970, 28 pp. 

17. Yeager, E. Acousto- 
Electrochemical Effects in Electrode 
Systems. Trans. Symp. on Electrode 
Processes. John Wiley & Sons, Inc., New 
York, Ch. 6, 1959, pp. 145-159. 

18. Yeager, J., J. P. Cels, E. 
Yeager, and F. Hovorka. I. Codeposition 
of Nickel and Hydrogen From Simple 
Aqueous Solutions. J. Electrochem. 
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19. Yeh, Y. Localized Fluid Flow 
Measurements With He-Ne Laser Spectro- 
meter. Appl. Phys . Letter, v. 4, No. 10, 
1964, pp. 176-178. 



INT. -BU. O F MINES, PGH., PA. 26720 



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